Gram-negative enteropathogenic bacteria, including Yersinia, Salmonella, Shigella and both enteropathogenic and enterohemorrhagic E. coli, cause a wide variety of gastero intestinal diseases in humans. All of these pathogens harbor an essential virulence factor called the Type III Secretion System which delivers bacterial toxins directly into host cells. The Type III Secretion System produces a structure in the bacterial membrane with a needle-like appendage extending out from the bacterial surface through which toxins are secreted. The secretion system also produces a pore in the host plasma membrane, termed the translocon, which allows access of the toxins to the host cell cytosol. It is not known how the needle on the bacterial surface and the translocon in the host cell membrane cooperate to enable toxin delivery. In an effort to determine the requirements for a productive needle-translocon interaction during infection I have isolated mutations in the needle protein of Yersinia pseudotuberculosis (YscF) that are incapable of delivering toxins into the host cell, but retain the ability to secrete toxins out of the bacteria into the extracellular space. In this proposal, I aim to use both WT Yersinia and the yscF mutants to: 1) determine if the needle is in direct contact with the translocon during infection, 2) determine if the needle-translocon interaction is required for pore formation in the host cell and 3) map the regions in YscF and the partner protein that are required for needle-translocon interactions. Characterization of the bacteria-host cell interactions using both WT and mutant Yersinia will provide insight into the mechanism of toxin delivery into host cells used by Y. pseudotuberculosis. This virulence mechanism is conserved amongst other enteric pathogens, and thus this work will be directly applicable to the study of disease caused by Salmonella, Shigella and E. coli, and will uncover candidates for targets of anti-microbial agents.